A number of materials are useful in medical, dental, biotechnology or veterinary applications because they are biocompatible in the sense that they do not present a serious rejection reaction when used as a biological implant or prosthesis or as in vivo or in vitro growth substrate. One difficulty with the use of such materials in veterinary, dental or surgical applications involves the fact that many such materials, including ceramics, metals, plastics, and composites, are rigid and hard. In many instances, this necesitates that the prosthesis or implant be shaped by grinding, sawing or otherwise during the surgical procedure in order to assure proper fit. E. Fischer-Brandies, "The Resorption of the Alveolar Ridge", Quintessence International, Vol. 12, 1985, 827-831 at 828. Alternatively, it may be necessary to provide a plurality of different sizes and shapes of implants or prostheses so that after surgery has begun, the surgeon can select the proper size and shape to assure the desired fit. These methods in general involve some additional surgical risk and increased costs since the surgical procedure is prolonged during the shaping and/or selection of the implant or prosthesis.
To eliminate the necessity for such shaping or selection, some types of dental or surgical implant or prosthesis procedures have employed a mass of particles in which each particle is typically on the order of a few microns up to a few millimeters in size. One such technique involves combining a mass of ceramic particles with a material which cures or sets to form a hard body, such as a polymerizable bonding material, as described in U.S. Pat. No. 4,097,935 issued July 4, 1978 to Jarcho. This technique, however, may be unacceptable when it is desired to have an implant with an enhanced number of sites available immediately for bone or tissue ingrowth, or when some amount of flexibility of the implant is desired, or when it is desired to minimize or eliminate shrinkage and/or compaction as the bonding material disintegrates or is resorbed. An implanted hard body can, particularly under condition of stress such as caused by mastication, rupture surrounding soft tissue, creating potential infection and bacteria growth sites. The Jarcho patent also discloses increasing the porosity of a sintered non-porous body by drilling or machining holes. The Jarcho patent does not disclose interconnectable or flexibly connected particles.
In other applications, particles are placed in the desired location, without a curable or setting bonding material, and the tissue of the host is allowed to grow into the implant material to eventually provide structural integrity. One such technique is used in alveolar ridge augmentation. In this technique, particles of a ceramic, often hydroxylapatite, can be injected, preferably in blood or saline solution, after suitable surgical preparation, by a syringe. Injection by syringe is possible because the particles are of a fluidizable size, i.e. are sufficiently small that, en masse, they are substantially fluid-like. The fluid-like characteristics of such particles allows not only ease of implantation but also permits the mass of ceramic material to be formed in a desired shape. Unfortunately, the fluid-like character of this material also requires particular care in surgical technique and, even with the best known technique, sometimes results in migration of the particles, i.e. movement to a location other than that desired. Such migration is a particular problem when the implant site is subjected to mechanical stress. Because tissue growth of the host into the implant material takes time, the patient must refrain from stressing the fluid-like implant and, in the case of alveolar ridge augmentation, this often means a soft or liquid diet for an extended period and often, containment with a stent. Review of Clinical Experiences, Supplement No. 2 S67 through S75.
As noted above, ceramic particle delivery has been attempted by mixing ceramic particles with saline or blood to form a slurry that will, to some degree, hold its position after placement. Victor J. Matukas "Newer Clinical Applications of Durapatite" at p. 22 in Alveolar Ridge Augmentation in Edentulous Patients. Another method which has been used in an attempt to minimize particulate migration is the encapsulation of particles within a tube-like structure as described in R. K. Gongloff "Comparison of Collagen Container and Uncontained Implants of Hydroxylapatite," Journal of Dental Research, Vol. 65, p. 336 (abstract only). This method, however, imposes a barrier between the particles and the host tissue relatively impermeable to easy ingrowth of tissue and can result in some retardation of tissue ingrowth into the mass of particles. The surgeon must pack the tube before or during the procedure or must have a variety of packed tubes to enable selection of the proper size. The tube is also subject to rupture with consequent loss of material.
Applications such as alveolar ridge augmentation involve two somewhat competitive considerations, viz., provision of mechanical strength, and provision of ingrowth sites, such as pores between or through the particles. Up to now, the primary means for anchoring implanted particles has been growth around the exterior of the particles. The need for tissue ingrowth into the implant material has prompted development of ceramic particles having a porous structure to provide sites for tissue ingrowth. However, provision of ingrowth sites has, up to now, invariably had a deleterious effect on the strength of the material. The attempt to provide acceptable ingrowth sites is illustrated in U.S. Pat. No. 3,890,107 issued June 17, 1975 to White et al. and 3,929,971 issued Dec. 30, 1975 to Roy, which disclose a ceramic particle constructed so as to have a plurality of pores. Particles with this type of porosity have a typical crushing strength of about 0.8 pounds (0.4 kg) when provided in a 20-25 mesh size. This crushing strength is substantially less than the crushing strength of particles which do not have such a highly macro-porous nature, which are typically on the order of about 5 pounds (2.3 kg) and up to 15 pounds (6.8 kg) or more. Further, the size and shape of the pores formed by this method are determined by the structure of marine life skeletal material which forms a basis for the final form of the particles and thus are limited to whatever forms happen to be found in nature. These materials are not adapted to solve the problem of implant migration.
Another method of providing porosity involves adjustment of reaction conditions during preparation of particles. It is possible to affect total microporosity volume in a ceramic by adjusting processing conditions such as sintering temperature and pressure, as described in U.S. Pat. No. 4,503,157 issued Mar. 5, 1985 to Hatahira. Such methods are ineffective to produce any desired pore shape or size and, further, cannot be used to affect pore characteristics without also affecting other ceramic characteristics such as crystal size. Another method of providing porosity includes tumbling the particles to produce particle agglomeration prior to sintering. It is particularly difficult in this method to adequately control pore size and density.
Accordingly, there is a need for a biocompatible article which can be implanted in tissue which has an amount of moldability or shapability, and yet is not subject to substantial migration from the implantation site or compaction after implantation. There is further a need for a biocompatible article which can be readily molded or formed to the desired size and shape during implantation. Additionally, there is the need for a biocompatible material which has high strength and is resistant to migration from the implant site when subjected to mechanical stress. Also, there is a need for a biocompatible implant material which is amenable to tissue ingrowth but which has a high crushing strength.